Oecumenius

And he says that those who wish to wrong them are defended by fire going forth from their mouths, and they have the authority to shut the sky so that no rain falls during the days of their prophecy. This is among the most probable; for if one of them had such authority, Elijah the Tishbite, even before completing the ministry of Christ, how unlikely would it be for him to precede Christ?

Commentary on Revelation ↗

Bede

And if anyone wants to harm them, fire proceeds from their mouth, etc. If anyone harms the Church, he is consumed by the fire of judgment, reciprocating the harm. For all who take the sword will perish by the sword (Matthew XXVI). Hence, the Chaldean flame, which was cast on the children of God, consumed the ministers of impiety themselves. Or spiritually, by the prayers of the Church's mouth, one is consumed by spiritual fire to be changed for the better. Coals of fire, he says, you will heap upon his head (Romans XII).

Commentary on Revelation ↗

Alcuin of York

And if any man will hurt them, fire shall come out of their mouths, and shall devour their enemies. And if any man will hurt them, in this manner must he be slain. What is symbolized by fire but holy preaching kindled by the firebrands of the Holy Spirit? It is not surprising if preaching makes the wicked die, since the apostles says, We are the good odour of Christ unto God in them that perish, etc. [2 Cor. …

Commentary On Revelation ↗

Victorinus of Pettau

"If any man will hurt them, fire proceedeth out of their mouth, and devoureth their enemies." That fire proceedeth out of the mouth of those prophets against the adversaries, bespeaks the power of the world. For all afflictions, however many there are, shall be sent by their messengers in their word. Many think that there is Elisha, or Moses, with Elijah; but both of these died; while the death of Elijah is not heard of, with whom all our ancients have believed that it was Jeremiah. …

Commentary on the Apocalypse of the Blessed John ↗